My invention relates to improvements in a control system such as that shown in European Patent Application No. 0027672 published Apr. 29, 1981, by Van Doorne's Transmissie B.V. My invention may be applied also to the control system shown in U.S. Pat. No. 4,152,947, which is assigned to Van Doorne's Transmissie B.V.
In a control system of known design, examples of which are shown in the two references identified in the preceding paragraph, adjustable cone sheaves are used to obtain an infinite variation in torque ratio wherein cone pulleys mounted on a primary shaft are displaceable by means of a primary pressure servo and wherein companion cone pulleys are mounted on a secondary shaft and are axially displaceable by means of a secondary pressure servo thus effecting an infinite variation in torque ratio.
The control system for actuating the servos includes a pressure pump, a main control valve and a transmission ratio control valve. The main control valve consists of a valve element disposed in a valve bore which is acted upon at one end by a spring force that varies in magnitude depending upon the engine throttle position. That force is opposed by a governor pressure force that is dependent upon the speed of the primary shaft. An increased throttle setting tends to move the control valve for the secondary servo toward an overdrive position. That causes the pressure on the secondary servo to decrease. This occurs as the primary servo pressure is increasing.
The force on the control valve that establishes the primary servo pressure is altered by a brake valve when the vehicle is in condition for low speed operation under coast conditions thereby allowing hill braking at low engine speeds.